This invention relates to coextruded films comprising vinylidene chloride methyl acrylate copolymer (VDC-MA), and particularly films which contain a layer of another polymeric material which has a melting temperature higher than the melting temperature of the VDC-MA.
As used herein, "film" means and includes films, which generically encompass thicknesses up to about 20 mils, sheets, which generically encompass thicknesses greater than about 20 mils, and extruded tubes of all thicknesses.
The invention especially related to the extrusion of a VDC-MA which is sensitive to a first set of thermal conditions at which it is processed, with a second polymeric composition which is typically processsed at a second set of more intense thermal conditions (higher temperature). Where the second set of thermal conditions is sufficiently intense, the VDC-MA can be adversely changed and degraded as a result of those conditions, by use of conventional extrusion methods.
In the present view of the inventors, VDC-MA is preferable to vinylidene chloride vinyl chloride (VDC-VC) copolymer in that films containing a VDC-MA layer provide better barrier properties than corresponding films containing a VDC-VC layer. Therefore, VDC-MA does have some properties that make it preferable over VDC-VC. However, extrusion of VDC-MA is more difficult than extrusion of VDC-VC, even under ordinary conditions, because it is more susceptible, than VDC-VC, to that degradation in the extrusion processing equipment which produces specks of black carbonized material in the extruded film. Accordingly, under conditions where it is difficult to extrude VDC-VC, one would expect even greater difficulty in extruding VDC-MA. So, the probability of success with VDC-MA under those conditions would be lessened, whereby a simple substitution of VDC-MA for VDC-VC would be expected to encounter greater difficulty. With respect to VDC-MA, reference is hereby made to commonly owned, concurrently filed, U.S. applications Ser. Nos. 07/458,483, 07/458,487, 07/458,488 and 07/458,490, which are hereby incorporated herein by reference.
Since coextruding a plurality of polymeric melt streams, in a desired relative structural relationship, at a single die exit is commonly practiced, it would be desirable to coextrude vinylidene chloride copolymers with a wide variety of other polymeric compositions. In particular, it would be desirable to be able to coextrude VDC-MA with other polymers which are normally coextrudable only at temperatures which are so high that VDC-MA loses its barrier or other favorable properties. Reference is made to commonly owned and concurrently filed U.S. application, Ser. No. 07/458,486 which is hereby incorporated herein by reference.
U.S. Pat. No. 3,558,330 Widiger et al relates to simultaneous extrusion of VDC-VC with other polymeric materials. They include a table of structures, at least one of which cannot be "coextruded" because one of the components, namely MYLAR is, by definition, already in film form. While Widiger et al generically disclose vinylidene chloride copolymer, al of the examples are specific to VDC-VC, whereby the application of the properties taught therein can only be attributed to VDC-VC.
However, it is well known that vinylidene chloride copolymers are sensitive to the thermal conditions to which they are exposed, as disclosed in subsequent U.S. Pat. No. 4,379,117 Baird Jr. et al. Baird Jr. et al suggest using hollow cavities in the die, to be filled with circulating liquids at an appropriate temperature to protect the vinylidene chloride copolymer from overheating. While Baird Jr. et al mention a plurality of vinylidene chloride comonomers, they do not suggest methyl acrylate comonomer, or any means of protecting the vinylidene chloride methyl acrylate copolymer.
It is an objective of this invention to provide multiple layer coextruded films including a first layer of VDC-MA, and a second layer of another polymeric composition which is typically extrusion processed at a higher temperature than the VDC-MA.
It is a more specific objective to provide multiple layer coextruded films wherein the second layer comprises a polymeric composition which is typically extrusion processed at a temperature at least about 50.degree. C. higher than the normal processing temperature of VDC-MA.
It is yet another object to provide methods and systems of extruding wherein the VDC-MA is exposed to less thermal stress than is characteristic of prior art processes.